The present invention relates to magnetic resonance imaging systems, apparatus and procedures. In magnetic resonance imaging, an object to be imaged as, for example, a body of a human subject, is exposed to a strong, substantially constant static magnetic field. The static magnetic field causes the spin vectors of certain atomic nuclei within the body to randomly rotate or xe2x80x9cprecessxe2x80x9d around an axis parallel to the direction of the static magnetic field. Radio frequency excitation energy is applied to the body, and this energy causes the precessing atomic nuclei to rotate or xe2x80x9cprecessxe2x80x9d in phase and in an excited state. As the precessing atomic nuclei relax, weak radio frequency signals are emitted; such radio frequency signals are referred to herein as magnetic resonance signals. Different tissues produce different signal characteristics. Furthermore, relaxation times are a dominant factor in determining signal strength. In addition, tissues having a high density of certain nuclei will produce stronger signals than tissues with a low density of such nuclei. Relatively small gradients in the magnetic field are superimposed on the static magnetic field at various times during the process so that magnetic resonance signals from different portions of the patient""s body differ in phase and/or frequency. If the process is repeated numerous times using different combinations of gradients, the signals from the various repetitions together provide enough information to form a map of signal characteristics versus location within the body. Such a map can be reconstructed by conventional techniques well known in the magnetic resonance imaging art, and can be displayed as a pictorial image of the tissues as known in the art.
The magnetic resonance imaging technique offers numerous advantages over other imaging techniques. MRI does not expose either the patient or medical personnel to X-rays and offers important safety advantages. Also, magnetic resonance imaging can obtain images of soft tissues and other features within the body which are not readily visualized using other imaging techniques. Accordingly, magnetic resonance imaging has been widely adopted in the medical and allied arts.
The magnets used in MRI imaging must provide a magnetic field that is strong and uniform. Preferably, the magnet of the MRI imaging apparatus has a patient-receiving space capable of receiving the torso of a normal human being, and provides a field strength of preferably at least about 3 kilogauss or more. This field desirably is uniform to within about a few parts in 106 or better in an imaging volume at least about 25 cm in diameter in the patient-receiving space.
The required fields can be generated by a so-called air-core solenoidal superconducting magnet. These magnets have coils positioned along a horizontal axis and a tubular central bore inside the coils. The patient is placed inside this tubular bore while he or she is being imaged. Although magnets of this type can provide acceptable images, they subject the patient to an intensely claustrophobic experience for the duration of the imaging procedure. Moreover, these magnets inherently require that the imaging procedure be conducted with the patient in a horizontal orientation, lying prone or supine on a bed.
More advanced magnetic resonance imaging magnets as described, for example, in certain embodiments of commonly assigned U.S. Pat. No. 6,014,070 (xe2x80x9cthe ""070 patentxe2x80x9d) provide a ferromagnetic frame with upper and lower pole support plates connected to one another by a set of columns disposed around the periphery of the pole support plates. Ferromagnetic poles projecting upwardly and downwardly from the pole support plates along a vertical axis define a patient-receiving gap. A patient may be disposed in the gap and subjected to a magnetic flux directed through the poles. The columns carry magnetic flux in a return flux path between the poles. As disclosed in the ""070 patent, the columns can be configured to provide a very open environment around the patient receiving gap, so that the patient is not subjected to a claustrophobic experience and so that the patient is readily accessible to medical personnel and also can provide a strong, highly uniform magnetic field for high-quality imaging. In certain embodiments disclosed in the ""070 patent, the poles are elongated so as to provide the uniform magnetic field over a similarly elongated region. However, magnets of this type still require the patient to assume a generally horizontal orientation.
Proposals have been advanced for magnetic resonance imaging magnets with ferromagnetic poles spaced apart from one another along a horizontal axis. As disclosed in commonly assigned U.S. Pat. No. 6,414,490 (xe2x80x9cthe ""490 patentxe2x80x9d), the disclosure of which is hereby incorporated by reference herein, a particularly desirable arrangement for such a magnet includes a generally cubical structure having a pair of opposed, vertically extending side walls and poles projecting inwardly from the side walls along a horizontal pole axis. The poles define vertically extending pole faces and a patient receiving space between these pole faces. The frame also includes upper and lower flux return structures extending between the side walls. As disclosed in greater detail in the ""490 patent, a patient can be positioned within the patient receiving space in a variety of orientations, ranging from a fully horizontal orientation reclining on a bed to a sitting or standing orientation in which the long axis of the patient""s torso extends substantially vertically. Moreover, the patient can be moved so as to align various regions of the patient""s body with the pole axis and thus bring various regions of the body into the imaging volume. In a particularly preferred arrangement taught in the ""490 patent, the upper and lower flux return structures define apertures and the patient support is arranged to move the patient upwardly and downwardly. For example, a patient in a standing position can be elevated so that his head projects into or through the aperture in the upper flux return structure while his legs and feet are disposed in the imaging volume, near the polar axis. The patient can be lowered so as to place his head in the imaging volume, near the polar axis while his feet project downwardly into or through the aperture in the lower flux return structure. Magnets according to the preferred embodiments taught in the ""490 patent provide a unique combination of versatility in positioning and a high quality magnetic field.
However, still further improvement would be desirable. In particular, the range of motion required to position any portion of a standing, normal size human adult within the imaging volume may be reduced. In addition, the total vertical clearance necessary to house a magnet that is capable of moving a standing patient through such entire range of motion may be also reduced within the limits that are currently required. Thus, it is desirable to have a magnet resonance imaging apparatus that requires a relatively low vertical clearance space and reduced range of motion to obtain a full body scan of a patient.
In accordance with an embodiment of the present invention, a magnetic resonance imaging apparatus comprises a frame including a pair of side walls extending in a vertical direction. A pair of elements project from the side walls and are spaced apart along a horizontal pole axis so as to define a patient-receiving space therebetween. The apparatus further includes upper and lower flux return members extending between the side walls. The flux return members each have a width dimension extending substantially transverse to the direction of the side walls and the horizontal pole axis and a vertical dimension extending substantially parallel to the direction of the side walls. The vertical dimension is desirably substantially smaller than the width dimension.
Further in accordance with this embodiment, it is desirable to have the ratio of the width to vertical dimension to be at least 1.5:1 and most desirably to be 2:1, 4:1, 5:1 or more.
This embodiment may further desirably include an imaging volume defined by the elements such that the vertical dimension of the imaging volume is larger than the horizontal dimension of the imaging volume along the pole axis.
In accordance with another embodiment, a magnetic resonance imaging apparatus preferably comprises a pair of substantially elliptical elements that are spaced apart from each other along a horizontal pole axis so as to define a patient-receiving space therebetween. The magnet is operable to direct flux between the elements.
Further in accordance with this embodiment, the apparatus desirably includes a pair of extending side walls. Each of the side walls abut each of said elements and form a surface from which the elements project.
The apparatus further desirably includes at least one upper flux return member and at least one lower flux return member that extend between the side walls.
In yet another embodiment a magnet resonance imaging apparatus comprises a magnet defining a substantially horizontal field axis and an imaging volume. The imaging volume is elongated in a vertical direction so that a vertical dimension of the imaging volume is greater than a horizontal dimension of the imaging volume transverse to the field axis. The apparatus further includes a patient support capable of supporting a human patient with the long axis of the patient""s torso in a substantially vertical orientation.
In accordance with this embodiment the patient support is capable of moving the patient upwardly and downwardly so as to align different regions of the patient with the imaging volume.
Apparatus implemented in accordance with this embodiment desirably define an imaging volume having a vertical dimension of approximately 2 feet and most desirably of at least 3 feet.
The apparatus may further desirably include upper and lower flux return members extending between a pair of vertically extensive side walls spaced apart from each other. Each of the flux return members having a vertical dimension V and a horizontal dimension H, wherein H is greater than V. In a most preferred embodiment the ratio of H to V is at least 2:1.
Apparatus implemented in accordance with the principles and embodiments of the present invention advantageously allow for a magnet resonance imaging magnet that requires substantially less vertical space for installation. This advance over the prior art allows installation of such magnets in facilities that include a vertical clearance of about 9 feet or less. This results in such apparatus being available for installation at a lower cost in facilities previously thought unsuitable for installation of MRI imaging apparatus of the type manufactured and marketed under the trademark Stand-Up MRI by Fonar Corporation.
Apparatus embodying the principles of the present invention further advantageously provide a relatively large vertical space for entry and exit of the patient and further enhances the open feel of the magnet.
Furthermore, the smaller vertical dimension of the flux return members advantageously allow for the poles of the magnet to be of a larger diameter or be elongated in the vertical dimension. This results in a larger imaging volume which allows for almost full body imaging. In addition, where elongated pole members are employed a full-body scan can be obtained by vertical adjustment of the patient.
Apparatus implemented in accordance with the present invention allow for methods of conducting magnetic resonance imaging comprising providing a static magnetic field directed along a horizontal field axis. The field is suitable for imaging within an imaging volume having a vertical dimension greater than the horizontal dimension of the imaging volume in a direction transverse to the field axis. A patient is disposed within the imaging volume such that the patient""s torso is oriented substantially vertically. The static magnetic field is then used to acquire magnetic resonance image data from a portion of the patient""s anatomy.